Recent years, syringes have been frequently used in which a liquid, such as a liquid medicine, is prefilled in the syringe. Such prefilled syringes may be filled with a solvent for solving a medicine in a vial.
For insertion of a gasket into the prefilled syringe, a method called vacuum capping is generally used. In the method, a barrel comprising a sealed distal end opening is filled with a medicine, the gasket is disposed at an opening portion of the barrel in a reduced-pressure atmosphere (vacuum atmosphere), and then the gasket is inserted into the barrel upon increasing the reduced-pressure atmosphere to a normal pressure state. When a plunger is mounted to the gasket, it is difficult to mount the gasket to the opening portion of the barrel. Accordingly, the capping work is performed on the gasket without the plunger. Therefore, the prefilled syringe generally requires subsequent mounting of the plunger. For easy mounting of the plunger to the gasket, prevention of liquid leakage during mounting of the plunger, and inhibition of internal pressure rise, a mounting mechanism has been generally employed in which the plunger is provided with an external threaded portion and the gasket is provided with an internal threaded portion, and the plunger and the gasket are mounted to each other by threaded engagement.
As described above, in the prefilled syringe filled with the solvent, the plunger mounted to the gasket is operated to be drawn for dissolving the medicine in the vial in the solvent or recovering the dissolved medicine. However, when the plunger is drawn, there is a risk that the plunger may be separated from the gasket, and the medicine cannot be dissolved or the dissolved medicine cannot be recovered.
Prefilled syringes that have a mechanism for preventing such separation of the gasket are disclosed in Japanese Patent Application Publication No. 2009-142508 A (“Patent Literature 1”) and Japanese Patent Application Publication No. 2011-72394 A (“Patent Literature 2”). It is noted that both Patent Literature 1 and Patent Literature 2 have been proposed by the applicant of the present application.
A syringe 10 of Patent Literature 1 comprises a gasket 3, a barrel 2, and a plunger 4. The plunger 4 comprises a helical rib 44 provided on an outer surface of a head portion 42, and the gasket comprises a helical valley portion 33 threadedly engaging with the helical rib, a plunger-retaining annular rib 34 positioned on the distal side near the helical valley portion 33, and an accommodation portion 32 for a part of the head portion of the plunger at which the helical rib is formed. The annular rib 34 comprises a rib absent portion 35 for guiding, to the accommodation portion, the helical rib reaching the annular rib as threaded engagement is advanced between the helical rib of the plunger and the helical valley portion of the gasket. The annular rib 34 prevents removal of the plunger from the gasket, retaining the helical rib 44 in the accommodation portion 32. A syringe 10 of Patent Literature 2 comprises a gasket 3, a barrel, and a plunger. The plunger comprises a helical rib provided at a head portion, and the gasket comprises a helical valley portion for threaded engagement with the helical rib, and an accommodation portion 32 for a part of the head portion of the plunger at which the helical rib is formed. The helical valley portion comprises helical crest portions 36, helical valley portions 33 formed between the helical crest portions 36, and a plunger retaining protrusion portion 34 formed at a terminal end part of the helical valley portion 33 and gradually protruding toward the accommodation portion 32. The protrusion portion 34 makes entrance of the helical rib of the plunger 4 into the terminal end part of the helical valley portion 33 difficult when the plunger is turned backward after being mounted to the gasket, and the protrusion portion 34 effectively prevents removal of the plunger from the gasket.